Silicon crystallization is a step that is often used in the manufacture of thin-film transistor (TFT) active-matrix LCDs, and organic LED (AMOLED) displays. The crystalline silicon forms a semiconductor base, in which electronic circuits of the display are formed by conventional lithographic processes. Commonly, crystallization is performed using a pulsed laser beam shaped in a long line. In this process, a thin layer of amorphous silicon on a glass substrate is repeatedly melted by pulses of laser radiation while the substrate (and the silicon layer thereon) is translated relative to a delivery source of the laser-radiation pulses. Melting and re-solidification (re-crystallization) through the repeated pulses take place until a desired crystalline microstructure is obtained in the film.
Optical elements are used to form the laser pulses into a line of radiation, and crystallization occurs in a strip having the width of the line of radiation. Every attempt is made to keep the intensity of the radiation pulses highly uniform along the line. This is necessary to keep crystalline microstructure uniform along the strip. A favored source of the optical pulses is an excimer laser, which delivers pulses having a wavelength in the ultraviolet region of the electromagnetic spectrum. The above described crystallization process, using excimer-laser pulses, is usually referred to as excimer-laser annealing (ELA).
FIG. 1A and FIG. 1B are respectively “long-axis” and “short-axis” views schematically illustrating one embodiment 10 of prior-art excimer laser annealing apparatus a laser beam having transverse axes perpendicular to each other is delivered to the apparatus from an excimer laser (not shown). The beam is longer in one of the transverse axes than in the other. For this reason these transverse axes are referred to by practitioners of the art as the long-axis and the short axis.
In the long-axis, as depicted in FIG. 1A, the laser beam, bounded by rays 14, passes through a short-axis homogenizer 16 including spaced apart arrays 18A and 18B of cylindrical lenses 19. These lenses have positive optical power in the short-axis only, having zero optical power in the long axis. After traversing the short-axis homogenizer the beam next traverses a long-axis homogenizer 20 including spaced apart arrays 22A and 22B of cylindrical lenses 23. These lenses have positive optical power in the long-axis only, having zero optical power in the short axis.
The beam then traverses, in sequence: a cylindrical lens 26 having positive optical power in the long-axis and zero optical power in the short-axis; a cylindrical lens 28 having positive optical power in the short-axis and zero optical power in the long-axis; a slit-aperture 30; a cylindrical lens 32 having positive optical power in the long-axis and zero optical power in the short-axis; and a cylindrical lens 34 having positive optical power in the short-axis and zero optical power in the long-axis.
Array 22A of homogenizer 20 divides the input beam into as many parts as there are cylindrical lenses in the array and converges these beam parts. The beam parts emerge diverging from lenses 23 in array 22B. Lenses 26 and 32 are configured and arranged such that each diverging beam-part from array 22A is spread to a long-axis pupil of apparatus 10 in which is located a layer 36 to be crystallized. The layer is supported on a glass pane (not shown) which in turn is supported on a translation stage (not shown). The effect of homogenizer 16 and lenses 26 and 32 is to create a flat-topped intensity distribution (intensity profile) along the long axis of the beam in the layer, as is known in the art. The length L of the beam on layer 36 is typically between about 300 mm and 1400 mm.
In the short axis, the input laser beam, bounded by rays 15 traverse short-axis homogenizer 16. Array 18A of homogenizer 16 divides the input beam into as many parts as there are cylindrical lenses 19 in the array and converges these beam parts. The beam parts emerge about collimated from lenses 19 in array 18B. Lenses 28 and 34 are configured and arranged such that each beam-part from array 22B is focused on layer 16. This provides that the short axis intensity distribution (intensity profile) on the layer is flat-topped in the manner of the long axis intensity distribution. The width W of the beam is typically between about 0.2 mm and 0.5 mm. The panel supporting layer 36 being crystallized is translated in the short axis direction relative to the focused beam as indicated in FIG. 1B by arrow A.
It should be noted that while arrays 16 and 20 are referred to above as homogenizer arrays, the arrays are actually only homogenizers when combined with condenser-lenses 28 and 26 respectively. A more accurate description would be lens-array pairs.
FIG. 2 is a graph schematically illustrating calculated intensity joules per cm (J/cm) as a function of short-axis distance for a beam in an example of apparatus 10. The beam has a short-axis width (FWHM) of about 0.4 mm. The vertical long dashed lines indicate typical translation increments of the beam width relative to layer 36 of FIG. 1B from one pulse to the next. In one preferred mode of operation, the translation speed of a layer-covered panel relative to the short-axis beam width is sufficiently slow that beam lines overlap, for example, by as much as 95% from one pulse to the next so any infinitesimal area receives a total of about 20 pulses. That is to say that the panel translates about one-twentieth of a beam-width from one pulse to the next.
The ELA process is a delicate process, and the error margin for the optimum energy density (OED) can be as small as 1%. It has also been observed that the OED can become smaller by more than this margin for an area of the film that has already been exposed to pulses.
A method for compensating for this OED variation during irradiation is described in U.S. Pat. No. 7,151,046. In this method, the intensity profile in the beam-width direction is formed into a step profile such that beam power increments lower after a predetermined number of the overlapping pulses. This stepped profile is achieved by placing absorbers at suitable positions in the beam path.
While the method of the '046 patent may be effective from the point of view of anticipating changes of OED with number of overlapping pulses, it is wasteful of laser energy. In the silicon recrystallization process a reduction of laser energy translates into a reduction in process throughput. Further, at the short wavelengths (less than 400 nm) of excimer laser radiation absorbers would be subject to damage, which at a minimum could alter the absorption values of the absorbers, and, accordingly, alter the step-profile. There is a need for an alternate method for achieving step-changes of power in the width direction of an excimer laser beam on a layer being crystallized.